1. Field of the Invention
This invention relates generally to a video signal processing apparatus and method for securing a copy protection effect, an apparatus for recording/reproducing the processed video signal and a record medium therefor.
2. Description of the Related Art
Digital technologies have now advanced to the point where end-users can easily obtain high quality video signal sources. For example, apart from pre-recorded tapes or software titles, there are digital broadcast terminals and DVDs (digital video disks) for providing the end-users with high-quality video signal sources. Although digital VCRs (video cassette recorders) are gradually becoming common, analog VCRs have been much more widely used than digital VCRs. Providers of such high-quality video signal sources need to pay careful attention to preventing the use of analog VCRs for the unauthorized copying of recorded materials which are afforded copyright protection.
Several copy protecting systems, such as an AGC (automatic gain control) pulse system and a color-stripe system developed by MACROVISION CORPORATION have been proposed for use in analog VCRs. In the AGC pulse system, in order to prohibit copying of a video signal, pulses with a larger level than the normal AGC reference level are placed in an AGC reference level detecting region of the video signal. In other words, pseudo-sync pulses are inserted in part of the vertical blanking intervals of the video signal, as shown in FIG. 13A. More particularly, as shown in the enlarged view of FIG. 13B, five of such pseudo-sync pulses are provided with a level larger by the amount p than the normal AGC reference level within each of the horizontal intervals of that part of the vertical blanking interval to be provided with the copy preventing signal.
In many analog VCRs, particularly intended for home-use, AGC is performed with 1H horizontal synchronous pulses in a vertical blanking interval, as shown in FIG. 13C. Thus, when pseudo-sync pulses with a larger amplitude than the horizontal synchronous pulses are provided in the 1H interval, as in FIGS. 13A and 13B, the AGC circuit acts as though the amplitude of the pseudo-sync pulses is the reference level and performs the AGC for such pulses. Thus, since the AGC level is detected as the level of the pseudo-sync pulses, the horizontal synchronous signal cannot be detected by amplitude separation. Consequently, the video signal cannot be correctly reproduced by the VCR. On the other hand, since the AGC system of television monitors is different from the AGC system of VCRs, the television monitors can normally display the reproduced pictures in spite of the presence of the pseudo-sync pulses.
However, the described AGC pulse system does not work effectively for some models of VCRs, such as, .beta.-type VCRs, 8-mm VCRs, and VHS-type VCRs provided with a long AGC time constant.
In order to solve the above problem, the so-called color stripe system has been proposed. In such color stripe system, the phase of the color burst signal is inverted four lines out of every 21 lines of a reproduced video signal. When the reproduced video signal including the color burst signal whose phase is partially inverted, as aforesaid, is recorded by another home-use analog VCR, an automatic phase controlling (APC) circuit of the analog VCR uses the color burst signal whose phase is partially inverted as the normal color burst signal for the respective lines. Thus, when the video signal is recorded/reproduced, the colors of the lines containing inverted color burst signals are inverted, with the result that the resultant video display has stripes S with inverted colors at every 21 lines, as shown on FIG. 14.
On the other hand, in conventional television receivers, a phase-locked loop (PLL) included in the APC to form a carrier for demodulating a color signal has a long time constant so that, even if the phase of the color burst signal is inverted for at most four successive lines out of every 21 lines, pictures displayed on the screen of a television receiver are not affected. However, some models of television receivers use phase locked loops (PLL) with a short time constant and, in such cases, stripes with inverted colors will be displayed on the screens thereof at the lines corresponding to the inverted color burst signals.
In order to solve the above-noted problem encountered in television receivers having phase-locked loops with a short time constant, the inventor of the present application has proposed another copy protection system in which the phase of the color burst signal is inverted for all lines of the effective screen in only parts of the signal. This system is effective for television monitors or receivers of the type in which the picture quality of the display is adversely affected when the color stripe system is used.
In the analog type home-use VCRs, a chrominance subcarrier with a frequency of 3.58 MHz for the NTSC system is converted to a low frequency band having a center frequency of 600 to 700 kHz. The resulting color conversion signal in a low frequency band and the luminance signal are frequency-multiplexed and recorded on a magnetic tape. Thus, in the home-use analog VCRs, the color signal is restricted to a frequency band that is very much narrower than the color signal band of television receivers and monitors, and is recorded on the magnetic tape in such narrow color signal band. When the color burst signal included in the recorded video signal is reproduced, since the frequency band is restricted, it tends to expand back and forth on the time axis in comparison with the original signal prior to recording. For example, FIG. 15A shows an original color burst signal a prior to recording, and FIG. 15B shows a color burst signal a.sup.1 that has been reproduced by an analog type home use VCR. In other words, when the original color burst signal a preceded by a horizontal synchronous signal h, as shown in FIG. 15A, is recorded on a magnetic tape and reproduced, the color burst signal a.sup.1 expands back and forth on the time axis as shown in FIG. 15B.
In view of the above noted characteristic, it has been proposed to partially invert the phase of the color burst signal for performing a copy protection function. For example, in each of the normal lines, 10 cycles of sub-carrier of a normal color burst signal are preceded by the horizontal synchronous signal h, as shown in FIG. 16A. In order to provide a copy protection effect, selected lines are each provided with six cycles of sub-carrier of the inverted color burst signal and with seven cycles of sub-carrier of the normal color burst signal whose phase is normal so as to provide a copy protection signal, as shown on FIG. 16B. On the screen, 17 lines with the normal color burst signal of FIG. 16A and 4 lines with the partially inverted color burst signal of FIG. 16B are alternately provided at intervals of 21 lines, as shown in FIG. 16C.
When the video signal including lines with the partially inverted color burst signal is reproduced, for example, by a VCR, the color synchronous circuit of the VCR does not follow the PLL, thereby causing deterioration of the picture quality. Thus, protection against unauthorized copying can be accomplished. Of course, even if this video signal is recorded/reproduced by another VCR, normal pictures cannot be obtained from the resulting second generation copy. However, television monitors detect and use the video signal including the partially inverted color burst signal as a nearly normal color burst signal, with the result that the television monitors display the original video signal including lines with the partially inverted color burst signal as a nearly normal signal.
However, since the system described above with reference to FIGS. 16A-16C uses the characteristic resulting from the narrow frequency band of the color signal in certain home-use VCRs, the copy protection effect is not sufficient for other VCRs having a wide color signal band, such as, S-VHS analog VCRs.
In order to improve the copy protection effect, a method in which the number of pulses of the inverted color burst signal is increased has also been considered. For example, as shown in FIG. 17, it has been considered to provide, in each of the lines containing partially inverted color burst signals, a copy protection signal composed of 10 cycles of sub-carrier of the inverted color burst signal and only 3 cycles of sub-carrier of the normal color burst signal. In such case, the copy protection effect is achieved for VCRs with a wide color signal band, but picture quality may be noticeably deteriorated in certain models of television monitors. For example, although colors are not inverted, in the noted models of television monitors, stripes with different densities may appear in the horizontal line direction of the screen or the densities of colors on the entire screen may vary.